The failure of an industrial control system can lead to costly downtime. There is expense involved in restarting a process along with the actual production losses resulting from a failure. If the process is designed to operate without supervisory or service personnel, all of the components in the process control system generally need to be fault-tolerant.
A fault-tolerant industrial process control system may employ a 1:1 redundancy system to synchronize the central processing unit (CPU) data in memory, where memory is maintained in an identical fashion in both a primary memory associated with a primary process controller and a secondary memory associated with a secondary process controller using an initial memory transfer followed by updates that are tracked changes to the primary memory image. As known in the art, a memory cache (cache) is a high-speed buffer for use with a lower-speed random access memory (RAM). CPU's without a memory cache (cache) express all changes to the RAM bus where they are captured (data and address). CPU's with write-through caching act in a similar manner. In contrast, CPUs with a cache commonly used for process control systems that need to react to process changes rapidly may not support write-thru caching.